1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method, and more particularly to an image forming apparatus and an image forming method for forming images by ejecting ink from nozzles.
2. Description of the Related Art
Inkjet type image forming apparatuses include an image forming apparatus which forms images by ejecting an ultraviolet-curable ink (so-called “UV ink”) onto a recording medium from nozzles provided in a print head. Conventionally, an image forming apparatus of this kind irradiates ultraviolet light (UV light) to all or a portion of the image formed on a recording medium after the end of a printing operation by the print head, so as to harden and fix the ink droplets which have been ejected onto the recording medium.
However, if ejected ink droplets land on the recording medium at a time interval that is shorter than the time required to permeate into the recording medium or to become fixed thereon, the ink droplets form one large ink droplet by combining and overlapping with each other before becoming fixed on the recording medium, or the ink droplets in which the dot shape is deformed permeate into the recording medium, and then there is a possibility of giving rise to bleeding, color mixing, and the like, so-called landing interference or droplet interference. Consequently, technology for preventing landing interference of this kind has been proposed (see Japanese Patent Application Publication Nos. 2001-310454, 2004-42548, and 2003-200564, for example).
Japanese Patent Application Publication No. 2001-310454 discloses a technology that an ultraviolet light irradiating unit provided in the print head irradiates ultraviolet light at the timing at which the ink droplets land on the recording medium.
Japanese Patent Application Publication No. 2004-42548 discloses a technology that a pre-hardening (preliminary hardening) operation is performed by irradiating ultraviolet light of a level sufficient to prevent mixing of ink droplets (dots) which have landed on the recording medium, whereupon ultraviolet light is subsequently irradiated again to perform main hardening operation.
Japanese Patent Application Publication No. 2003-200564 discloses a technology that an ultraviolet light source is provided on the back side of the print surface of a recording medium, ultraviolet light is irradiated from this light source to the recording medium. When a print head having nozzles is situated over the recording medium, irradiation of ultraviolet light to the nozzles is prevented by means of a shield plate which shields the ultraviolet light.
However, in the technology disclosed in Japanese Patent Application Publication No. 2001-310454, if the ultraviolet light is irradiated to the ink droplets on the recording medium, then a portion of the ultraviolet light is reflected and reaches the nozzles, thus causing the ink in the vicinity of the nozzle aperture (the ink in the vicinity of the nozzles) to harden. In particular, when ultraviolet light is irradiated from directly below the nozzles (in the ink ejection direction), the ink in the vicinity of the nozzles is liable to harden, and hence ejection faults such as nozzle blockages occur.
According to the technology disclosed in Japanese Patent Application Publication No. 2004-42548, if different nozzles (or print heads) eject ink droplets at a prescribed time delay with respect to each other, landing interference between ink droplets (dots) ejected from different nozzles can be prevented by performing pre-hardening between each droplets ejection. However, this technology is not considered about landing interference of the ink droplets ejected from the same nozzle. For example, if ink is ejected from the same nozzle in consecutive ejection cycles, the pre-hardening is not performed between these ejections, and hence landing interference occurs. In addition, the reflected portion of the ultraviolet light irradiated to the ink droplets on the recording medium is liable to harden the ink in the vicinity of the nozzles.
In order to resolve the problem of ink solidification in the vicinity of the nozzles, in Japanese Patent Application Publication No. 2003-200564, as described previously, an ultraviolet light source is provided on the back side of the print surface of a recording medium, ultraviolet is irradiated from this light source to the recording medium, and irradiation of ultraviolet light to the nozzles is prevented by means of a shield plate which shields the ultraviolet light when a print head having nozzles is situated over the recording medium. However, if ink is ejected from the same nozzle in consecutive ejection cycles, then the ultraviolet light remains shielded by the shield plate. Therefore, the ink droplets on the recording medium may not be hardened, and then landing interference may occur.